Pop in a good advert (3)
I believe the answer is:
dad
'pop' is the definition.
('dad' can be a synonym of 'pop')
'in a good advert' is the wordplay.
'in' says the answer is hidden in the clue.
'DAD' is hidden in the letters of 'good advert'.
(Other definitions for dad that I've seen before include "Relation" , "Popular father - a palindrome" , "Papa" , "Father, in short" , "Popular and informal father" .)